In a reciprocating engine (reciprocating-piston engine), except for some 2-cycle engines, there are air-intake valves and exhaust valves that open and close in synchronization with the rotation of the crankshaft. In this kind of reciprocating engine, the movement of the camshaft that rotates in synchronization with the rotation of the crankshaft (½ the rpm in the case of a 4-cycle engine) is transmitted to the air-intake valves and exhaust valves by rocker arms, and causes the air-intake valves and exhaust valves to move in a reciprocating motion in the axial direction.
Conventionally, castings (cast iron parts or aluminum die-cast parts) were used for the rocker arm assembled in the valve gear of this kind of engine. However, in recent years, manufacturing the rocker arms by pressing metal plate such as steel plate has been considered, and is being performed somewhat. The reason for this is that castings are heavy (in the case of cast iron parts) or that a lot of volume is required for securing sufficient strength (in the case of aluminum die cast parts), and that since they are typically manufactured using a lost-wax process, the manufacturing cost is high.
Prior art technology related to a metal plate rocker arm that takes this into consideration has been disclosed in Japanese patent publication No. Tokukai 2000-120411. FIGS. 4 to 10 show the invention related to the rocker arm and manufacturing method described in that disclosure. As shown in FIG. 4, this prior art rocker arm 1 comprises a pair of side-wall sections 2 that are nearly parallel with each other, a connecting section 3 and second connecting section 4 that connect the edges on one end in the width direction of these side wall sections 2 to each other, respectively. Also, a pair of circular holes 5 are formed in the middle section in the lengthwise direction of both of side wall sections 2 such that they are concentric with each other, and both of these circular holes 5 support both ends of a support shaft that supports the roller in engagement with the cam such that the roller rotates freely. Of the connecting section 3 and second connecting section 4, a fitting surface 6 that comes in contact with the base end of the valve stem is formed on one surface of the connecting section 3, and a fitting section 7 that comes in contact with the tip end of the lash adjuster is formed on the second connecting section 4. It is not shown in the figures, however, construction of forming a screw hole in the second connecting section and screwing an adjuster screw into that screw hole is also disclosed in Japanese patent publication No. Tokukai 2001-59407 and is well known in the art.
Of the fitting surfaces 6 and 7, fitting surface 6 is formed in the middle section in the width direction of the connecting section 3 on its one surface by causing this middle section in the width direction of the connecting section 3 to deform plastically in the direction of thickness such that it has a concave groove shape which is depressed more than the other sections of the connecting section 3. Also, there is a protruding section 8 having a trapezoid shaped cross section formed on the other surface of this connecting section 3 such that it protrudes in an embankment shape when the fitting surface 6 is formed. On the other hand, the fitting surface 7 is formed by plastically deforming the center section of the second connecting section 4 in the direction of thickness such that it is a spherical concave surface. In the case of construction using an adjuster screw, the tip end of the adjuster screw is formed in a spherical convex surface shape.
When manufacturing this kind of rocker arm 1, first, in a first process, a first blank plate 9 as shown in FIG. 5 is made. In other words, in this first process, a sufficiently rigid metal plate (flat plate or coil plate) such as carbon steel plate having a thickness of, for example, 3 mm to 4 mm is supplied between the punching die and receiving die of the press apparatus (not shown in the figure), and the first blank plate 9 is punched and formed between these dies.
As shown in FIG. 5(A), this first blank plate 9 has a diamond-like shape with rounded corners and with on one end in the lengthwise direction {right end in FIG. 5(A)} cut out, and has a thickness t9 {FIG. 5(B)}. The section having width W10 that is located in the center section in the width direction {up-down direction in FIG. 5(A)} of this first blank plate 9, that is in the section located further inward than the two dashed lines α in FIG. 5(A) (section in and near the center in the width direction) is the base section 10 that extends in the lengthwise direction {left-right direction in FIG. 5(A)} of this first blank plate 9. Also, on both sides in the width direction of this base section 10, there is a pair of triangular-shaped fin sections 11.
Next, as shown in FIG. 6(A), in a second process, a through hole 12 is formed in the center section of this first blank plate 9, to form a second blank plate 13. This through hole 12 has a large hand-drum shape, and there is a pair of partially arc-shaped tab sections 14 formed on both sides in the width direction of the through hole 12 and in the center section in the lengthwise direction of the blank plate 13 such that they protrude in a direction toward each other. Both of these tab sections 14 are formed in order to form the circular holes 5 (see FIG. 4 and FIG. 10) for supporting the ends of the support shaft, so that the roller supported (described later) rotates freely. Also, semi-circular notch sections 15 are formed in the four corners of the through hole 12. These notch sections 15 are formed in order that in the third process, when forming a curved section 16 (see FIG. 7) by bending the base section 10 such that it has a substantially arc-shaped cross section, the bending work can be performed more easily.
This second blank plate 13 is formed by supplying the first blank plate 9 between a punching die and receiving die that are assembled in the pressing apparatus (not shown in the figures), and punching out the aforementioned through hole 12 between these dies. The width W10 of the base section 10 of the first blank plate 9 and second blank plate 13 is greater than the space between the outside surfaces of the pair of side-wall sections 2 that are formed in the third process (described next), that is the width W17 of a first intermediate member 17 (see FIG. 7) (W10>W17). As the width W10 of the base section 10 is made greater than the width W17 of the first intermediate member 17 in this way, the space D14 between the pair of tab sections 14 is increased, and thus it is possible to maintain the life of the punching die used for punching the through hole 12. It is also possible to change the order for making the second blank plate 13 from what was described above.
The second blank plate 13 that is formed into the shape shown in FIG. 6 becomes the first intermediate member 17 in the third process as shown in FIG. 7. In this third process, the second blank plate 13 is supplied and strongly pressed between the pressure die and receiving die of the pressing apparatus (not shown in the figures) to bend the base section 10 of this second blank plate 13 and the fin sections 11, 11. Also, this second blank plate 13 becomes the first intermediate member 17 having a pair of side-wall sections 2, 2 on the left and right in the width direction, and curved sections 16 that connect the opposite end edges in the width direction {left and right direction in FIGS. 7(C) and 7(D)} of these side-wall sections 2, respectively. The curved sections 16 are formed into a semi-cylindrical shape such that they are non-continuous in the middle in the lengthwise direction {left and right direction in FIG. 7(A)} of the first intermediate member 17 that corresponds to the through hole 12. One of the curved sections 16 divided in two by the through hole 12 in this way, is provided on one end side {right end side in FIGS. 7(A) and 7(B)} and becomes the connecting section 3 (see FIGS. 4, 9 and 10) which comprises the fitting surface 6 coming in contact with the base section of the valve, and the other is provided on the other end side {left end side in FIGS. 7(A) and 7(B)} and becomes the second connecting section 4 (see FIGS. 4, 9 and 10) which comprises the fitting surface 7 coming in contact with the tip end of the lash adjuster. Also, in the case of construction that uses an adjuster screw, a screw hole is formed in this second connecting section 4.
As described above, the width W17 of the first intermediate member, which is the space between the outside surfaces of the pair of side wall sections 2, is less than the width W10 of the base section 10 of the first and second blank plates 9 and 13. In other words, in the first intermediate member 17, the curved sections 16, which play the role of a connecting section for connecting the end edges in the width direction of the pair of side wall sections 2, is formed into a substantially semi-cylindrical shape as shown in FIGS. 7(C) and 7(D). The curved sections 16 are formed into a semi-cylindrical shape in this way such that the width of the curved sections 16 is less than the width W10 of the base section 10 of the flat plate, which is the origin of the curved sections 16, so it is possible to make the width W10 of this base section 10 greater than the width W17 of the first intermediate member 17, which is the space between the outside surfaces of the left and right side wall sections 2 formed on the first intermediate member 17 (W10>W17), and thus it is possible to increase the space D14 between the aforementioned tab sections 14. The thickness t16 of the curved sections 16 of the first intermediate member 17 that was obtained from the third process described above and as shown in FIG. 7, is nearly the same as the thickness t9 of the first blank plate 9 (t16≈t9).
Pressing is performed in a fourth process, which will be described next, on at least an end side portion of the curved section 16 to increase the thickness, such that the end side portion becomes the fitting surface 6 that comes in contact with the base section of the valve stem. In this case, in order to obtain the desired thickness after the pressing process, it is necessary to regulate the shape and dimensions of the curved sections 16. In other words, the selection of shape and dimension on the curved sections 16 determines the thickness in the pressing process. Moreover, when forming the curved sections 16, the pair of side-wall sections 2 are formed at the same time in the intermediate blank 17. In other words, the fin sections 11 that are formed on both ends in the width direction of the first and second bland plates 9 and 13, and the tab sections 14 that are formed on the inside edge of the through hole 12 in the center section are stood up to form the pair of nearly parallel side-wall sections 2 as the curved sections 16 are formed.
In the fourth process, pressing is performed on the curved sections 16 of the first intermediate member 17 that was constructed as described above to form the second intermediate member 18 as shown in FIG. 8. In other words, in the fourth process, the curved sections 16 are pressed into a flat plate and the thickness is increased, and as shown in FIG. 8, the connecting section 3 and second connecting section 4 are made to have thickness t3 and t4 that are greater than the thickness t9 of the first blank plate {see FIG. 5(B)} (t9<t3, t4).
In the fourth process, the curved sections 16 of the first intermediate section 17 are set between the pressing die and the receiving die, and pressure is applied by cold forging to plastically deform the curved sections 16. As a result, a flat connecting section 3 and second connecting section 4 are formed. In this way, when plastically deforming the curved sections 16 to form the connecting section 3 and second connecting section 4, the thickness is increased to t3 and t4 in the curved sections 16 with arc-shaped cross section as they become the flat connecting section 3 and second connecting section 4, respectively. The process of transforming the curved sections 16 having an arc shaped cross section into the flat connecting section 3 and second connecting section 4, respectively, while at the same time increasing the thickness, can be performed easily using a pressing process that uses a press.
In the example shown in the figures, not only is the thickness of the connecting section 3 formed on the side of one end increased, but the thickness of the second connecting section 4 that is formed on the side of the other end is also increased. However, it is on the side of the connecting section 3, where the fitting surface 6 is formed to come in contact with the base section of the valve stem, that large stress is particularly applied when using the rocker arm. Therefore, it is not absolutely necessary to increase the thickness on the side of the second connecting section 4. If it is not necessary to increase the thickness, it is possible to simply plastically deform that curved section 16 to form the flat connecting sections. However, when the thickness of the connecting section 3 and second connecting section 4 are the same, it is possible to reduce the amount of processing and to be more cost effective.
In the fourth process, if the second intermediate member 18 is formed from the first intermediate member 17 by forming a relatively thick connecting section 3 and second connecting section 4, then next, in the fifth process, plastic deformation or cutting and grinding is performed on the connecting section 3 and second connecting section 4. In other words, as shown in FIG. 9, the fitting surface 6 that comes in contact with the base section of the valve stem (not shown in the figure) is formed on the connecting section 3. Also, the fitting section 7 that comes in contact with the tip end of the lash adjuster (not shown in the figure) is formed on the second connecting section 4. In this fifth process, the connecting section 3 of the second intermediate member 18 is set between the pressing die and receiving die of the forging machine (not shown in the figure), and by performing cold forging on the connecting section 3, a fitting surface 6 is formed having a concave groove shape as shown in FIGS. 9(A), 9(B) and 9(D) where the bottom surface is curved in a convex shape. Also, the second connecting section 4 is set between the pressing die and receiving die of a different forging machine (not shown in the figure), and by performing cold forging on this second connecting section 4, a spherical concave fitting section 7 as shown in FIGS. 9(A), 9(B) an 9(C) is formed. Formed in this fifth process is a third intermediate member 19 which is formed with a fitting surface 6 and fitting section 7 on the connecting section 3 and second connecting section 4 that are thicker than the first blank plate 9.
Next, in a sixth process, circular holes 5 are formed in the third intermediate member 19, that was obtained as described above, in the middle section of the pair of side-wall sections 2 using a press or a cutting process such that they are aligned with each other, to complete the rocker arm 1 as shown in FIG. 4 and FIG. 10. These circular holes 5 are for supporting both of the ends of the support shaft (not shown in the figures) that supports the roller (also not shown in the figures) such that it can rotate freely. In other words, when installed in the engine, a roller is supported around the middle section of the support shaft whose ends are supported in the circular holes 5 such that it can rotate freely, and the outer peripheral surface around this roller comes in contact with the outer peripheral surface around the cam that is fixed to the camshaft. Also, the base end face of the valve stem 20, which is the air-intake valve and exhaust valve, comes in contact with the fitting surface 6 (see FIGS. 1 to 3, 11 and 13 to 18 showing an embodiment of the present invention), and the tip end face of the lash adjuster (not shown in the figures) comes in contact with the fitting section 7. The tip end face of this lash adjuster is a semi-spherical convex surface, and this surface on the tip end fits with the fitting section 7 such that it can rock and move freely. With this kind of construction, the rotation of the camshaft is freely converted to rocking motion of the rocker arm 1. Also, in the case of construction using an adjuster screw, the spherical convex surface formed on the tip end of this adjuster screw comes in contact with the bearing surface, so that the rocker arm rocks freely around this point of contact.
A pair of guide surfaces 21 is formed on both sides in the width direction of one surface of the connecting section 3 of the rocker arm 1 such that they are located on both sides in the width direction of the fitting surface 6. More specifically, these guide surfaces 21 are formed on wall sections that continue on from the side-wall sections 2. Also, the base end section of the valve stem 20 that comes in contact with the fitting surface 6 is prevented from becoming separated from the fitting surface 6 in the width direction by this pair of guide-wall sections 21. Moreover, in the case of the prior art technology shown in FIG. 4 to 10, these guide surfaces 21 are flat parallel surfaces.
The rocker arm and manufacturing method described above not only make it possible to improve the strength and rigidity of the rocker arm, but by reducing the number of manufacturing process and the number of parts, it is possible to reduce costs, improve precision and simplify the equipment, and thus it is possible to manufacture a high quality rocker arm at low cost.
Incidentally, it is sometimes preferred to change the width of the rocker arm according to the parts that fit in the rocker arm, or in other words according to the width and diameter of the roller 30 (see FIGS. 19 and 20 described later), the lash adjuster and also the valve stem 20. Since the widths and diameters of the roller 30, lash adjuster and valve stem 20 are all different, it is sometimes preferred to change the width of the rocker arm to correspond with the width and diameter of these parts in order to make a good fit between these parts, the lash adjuster and the rocker arm. A rocker arm 1 as shown in FIGS. 19 and 20 and disclosed in Japanese patent publication No. Tokukai Hei 7-229407 is manufactured taking this point into consideration.
In the case of the rocker arm 1 shown in FIGS. 19 and 20, the side wall sections 2 is crooked, so that the width of the connecting section 3 that comes in contact with the base end section of the valve stem 20 is narrow, and that the width of the second connecting section 4 that comes in contact with the tip end section of the lash adjuster is wide. The space where the roller 30 is supported in the middle section between the side wall sections 2 is sized between the space for the connecting section 3 and the space for the second connecting section 4.
In the case of the prior art construction described above, the pair of guide surfaces 21 that are formed on both sides in the width direction of the fitting surface 6, are flat and parallel surfaces, so when the base end section of the valve stem 20 (see FIGS. 1 to 3) moves to a side in the width direction of the fitting surface 6 such that the outer peripheral surface of the valve stem 20 comes in contact with anyone of the guide surfaces 21, the contact area is broader (the length of the contact area is longer in the axial direction of the valve stem 20). As a result, the friction force acting on the point of contact between the outer peripheral surface of the valve stem 20 and that guide surface 21 is large, and as the rocker arm 1 rocks, the friction loss is large. This kind of increase in friction loss, is not desirable since it hinders improvement of performance such as engine fuel efficiency and power efficiency.
Also, as shown in FIGS. 19 and 20, in the case where the middle section of the left and right side-wall sections 2 are bent in order that the width of the connecting section 3 that comes in contact with the base end section of the valve stem 20 becomes narrow, a large tensile stress is applied to part of the metal plate of the rocker arm 1 in the bent sections 24 of these side-wall sections 2. As a result, it becomes easy for damage such as cracking to occur in and near the bent sections 24, and it becomes difficult to maintain the reliability and durability of the rocker arm 1.